Amother wants to invest $12,000 for her son education. she invests a portion in a bank called a cd account which earns 4% and the remainder in a savings bond that earns 7%. if the total interest earned after one year is $720.00, how much was invested in the cd account

The amounts are "a" on the CD at 4% and "b" on the bonds at 7%

now, she had 12,000, so we know whatever "a" and "b" are, they

a + b = 12000

now... what's 4% of a? well, (4/100) * a, or 0.04a
what's 7% of b? well  (7/100) * b or 0.07b

we know the total interest, that 4% and 7% amounts, add up to 720, thus

0.04a + 0.07b = 720

thus    

solve for "a", to see how much was invested in CD

what about "b"? well b = 12000 - a